A standard drum centrifuge has a housing in which a foraminous drum is rotated at high speed about its axis. A suspension is fed to the interior of the drum so that it is thrown centrifugally against the wall thereof. At first the suspension forms an annular body in the drum having an inner surface centered on the axis, then this annular body stratifies and the liquid phase passes through the drum and the solid phase stays behind on the inner surface of the drum as a filter cake. This inner layer of liquid passes radially outward through the layer of solids until same is substantially dry. As a rule the drum is filled and refilled several times until the filter cake builds up to a desired depth. Then this cake is washed by passing a liquid through it, and then it is centrifuged to an extremely low moisture content. Subsequently a hot gas can be passed through it to further dry it, and finally it is physically stripped out of the drum, same is regenerated, and the cycle is restarted.
It is known to form an end wall of the drum with an axially inwardly open bypass chamber that extends radially inward well past where the inner surface of the thickest filter cake would be. This chamber is separated from the interior of the drum by a mesh or other foraminous wall so that filtrate liquid can pass through it but the solid phase, that is the filter cake, cannot. As described in U.S. Pat. Nos. 4,101,421 and 4,184,951 as well as in European patent application 255,623 filed Jul. 11, 1987 by P. Franzen et al the bypass chamber is provided with its own drain system so that liquid can be drawn out of it independently of the liquid that is drained off outside the foraminous inner wall of the drum.
In this manner it is possible to operate the centrifuge in the normal manner without permitting any liquid to exit the bypass chamber until a fairly thick filter cake has built up. At this time the bypass chamber can be drained to draw off the filtrate liquid sitting atop the inner surface of the filter cake, thus speeding operation by not forcing this liquid to slowly percolate down through the filter cake. Later when the filter cake is being rinsed, the bypass chamber is again closed to ensure that all the rinse liquid passes through the filter cake.
The disadvantage of such as system is that it is fairly complex, what with the two separate drainage systems for the endwall and side-wall filtrate. Since both of these liquids are invariably routed to the same location, the extra cost of the dual drainage system is unwarranted.